1. Field of the Invention
The present invention relates to a hybrid magnetic substrate and a method for producing the same. In particular, the present invention relates to a hybrid magnetic substrate in which a magnetic substrate is bonded to a holding substrate through direct bonding and a method for producing the same.
2. Description of the Related Art
As devices utilizing a magnetic, there are optical isolators allowing light to pass only in a predetermined direction by using a magneto-optic effect and various kinds of magnetostatic wave devices using a magnetostatic wave propagating through a magnetic. In most cases, these devices utilize a magnetic as a magnetic substrate layered on another substrate or as a thin film formed on another substrate. In the present specification, an entire substrate composed of a substrate and a magnetic substrate or magnetic film layered on the substrate is referred to as a hybrid magnetic substrate.
For example, Japanese Laid-Open Patent Publication No. 2-232606 discloses a thin film wave guiding type optical isolator having, on a garnet single crystalline substrate, a garnet type thin film as an intermediate layer and another garnet type thin film as a waveguide layer made of a magneto-optic material. This construction is formed by a liquid phase epitaxial method.
FIG. 1 shows a construction of a conventional optical isolator 201. In this figure, a substrate 101 is made of, for example, a gadolinium-gallium-garnet (GGG) single crystal. The intermediate layer 104 is formed by a liquid phase epitaxial method. As the intermediate layer 104, yttrium-iron-garnet (YIG) type materials are used. An optical waveguide layer 102 is formed on the intermediate layer 104 by a liquid phase epitaxial method. As the optical waveguide layer 102, YIG type materials are also used. On the optical waveguide layer 102, an upper layer 103 is formed. As a material for the upper layer 103, ZnO is used, for example. The upper layer 103 is not indispensable. Instead of forming the upper layer 103, the upper face of the optical waveguide layer 102 may be exposed.
Light is confined in the optical waveguide layer 102 by providing the intermediate layer 104 having a refractive index smaller than that of the optical waveguide layer 102. Furthermore, the optical waveguide layer 102 is made of a YIG type material having a magneto-optic effect, so that it works as an optical isolator.
Regarding magnetostatic wave devices, for example, J. Castera (Journal of Applied Physics, 55(6), pp. 2506-2511, 1984) discloses a method for producing a magnetostatic wave device and various applications of the magnetostatic wave device. This publication describes a method for forming a thin film made of YIG on a GGG substrate by a liquid phase epitaxial method, and exemplifies filters, oscillators, delay lines, etc. as the applications of the magnetostatic wave device.
The above-mentioned conventional examples have the following problems:
In a method for forming a hybrid magnetic substrate by various kinds of epitaxial growth methods, only a film with a lattice constant matching with that of a substrate can be formed, so that the combination of the materials for the film and the substrate is limited. The crystal orientation of the film to be formed depends upon that of the substrate, which decreases the degree of freedom of forming a film having a preferred crystal orientation. Furthermore, even though any of the epitaxial growth methods such as liquid phase epitaxial growth, chemical vapor phase epitaxial growth, and molecular beam epitaxial growth is used, it has been difficult to obtain a magnetic film having characteristics superior to those of a bulk single crystal.
For layering substrates made of different materials, methods for fixing them with various kinds of adhesives can be used. However, according to the conventional methods using adhesives, the precision of thickness of interface layers bonding the respective layers to each other, heat resistance, chemical resistance, and reliability are not sufficient.